Interrogator and system employing the same

ABSTRACT

An interrogator and system employing the same. In one embodiment, the interrogator includes a receiver configured to receive a return signal from a tag and a sensing module configured to provide a time associated with the return signal. The interrogator also includes a processor configured to employ synthetic aperture radar processing on the return signal in accordance with the time to locate a position of the tag.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/292,209, entitled “Interrogator And System Employing The Same” filedOct. 13, 2016, issuing as U.S. Pat. No. 10,324,177 on Jun. 18, 2019which is a continuation of U.S. patent application Ser. No. 14/710,858,entitled “Interrogator And System Employing The Same” filed on May 13,2015, now U.S. Pat. No. 9,470,787 on Oct. 18, 2016, which is acontinuation of U.S. patent Ser. No. 13/443,594, entitled “InterrogatorAnd System Employing The Same”, filed on Apr. 10, 2012, issued as U.S.Pat. No. 9,035,774 on May 19, 2015, which claims the benefit of U.S.Provisional Application No. 61/474,056 entitled “Innovative, Novel, andUnconventional Integration of SAR-Like Processing Techniques with RFID,”filed Apr. 11, 2011, which application is incorporated herein byreference.

TECHNICAL FIELD

The present invention is directed to radio frequency identificationsystems and weapon systems, and method of operating the same.

BACKGROUND

Radio Frequency Identification (“RFID”) tags are finding wideapplication in inventory control and tracking systems. In these systems,a two-way communications link is established wherein readers (orinterrogators) interrogate RFID tags to respond with theiridentification and perhaps other ancillary information. The RFID tagscan be broadly separated into three categories, namely, active,semi-active and passive. The active RFID tags contain a battery andcommunicate on the reverse link (RFID tag to reader) using conventionalradio frequency (“RF”) communications techniques. The passive RFID tagstypically derive power from the reader using a diode rectifier and thenrespond on the reverse link using a backscatter modulation techniquethat modulates the apparent radar cross section of the device. Thesemi-active RFID tags employs properties from the other two in that ithas a battery, allowing it to respond to signals of lower amplitude thana passive RFID tag, however, its response is the same as a passive RFIDtag in that it employs a backscatter modulation technique. The mostcommon backscatter modulation approach is to modulate byshorting/opening an antenna of the RFID tag to vary the radar crosssection (“RCS”).

One major weakness with extant RFID systems is their poor sensitivitylimiting operation to fairly high signal-to-noise (“SNR”) regimes. Thisproblem has been addressed, in part, with major improvements insensitivity using knowledge of the target RFID tag's identification anda combination of coherent and noncoherent integration techniques. Asaddressed herein, further improvements are necessary to more accuratelylocalize the position of an RFID tag.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by advantageous embodimentsof the present invention, which includes an interrogator and systememploying the same. In one embodiment, the interrogator includes areceiver configured to receive a return signal from a tag and a sensingmodule configured to provide a time associated with the return signal.The interrogator also includes processor configured to employ syntheticaperture radar processing on the return signal in accordance with thetime to locate a position of the tag.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of an embodiment of a syntheticaperture radar processor;

FIG. 2 illustrates a graphical representation of the radial range ofRFID tags as a function of time;

FIG. 3 illustrates a graphical representation of a synthetic apertureradar image associated with three RFID tags;

FIG. 4 illustrates a graphical representation of a RFID backscatterspectrum;

FIG. 5 illustrates a graphical representation of a frequency steppingwaveform with a randomized step size to yield better temporal sidelobeperformance;

FIGS. 6 and 7 illustrate matched and mismatched filter responses,respectively;

FIGS. 8 and 9 illustrate graphical representations of a syntheticaperture radar image associated with three RFID tags;

FIG. 10 illustrates a block diagram of an embodiment of an active RFIDtag;

FIG. 11 illustrates a view of an embodiment of a radar silhouette of aship produced with an inverse synthetic aperture radar processor;

FIG. 12 illustrates a view of an embodiment of a synthetic length of aderived synthetic aperture radar antenna;

FIG. 13 illustrates a view of an embodiment of synthetic aperture radarprocessing;

FIGS. 14 and 15 illustrate views of embodiments of vehicles including areader;

FIGS. 16 and 17 illustrate views of embodiments of a vehicle and a bomb,respectively, including a reader separated into multiple sections;

FIGS. 18 to 20 illustrate graphical representations of a syntheticaperture radar image associated with three RFID tags;

FIG. 21 illustrates a view of an embodiment of a system to locate RFIDtags with synthetic aperture radar processing;

FIGS. 22 and 23 illustrate views of embodiments of a RFID system;

FIG. 24 illustrates a view of an embodiment of a reader; and

FIGS. 25 and 26 illustrate views of an embodiment of a RFID tag.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present disclosure is related to RFID systems and weapon systems.For exemplary weapons and weapons systems, see U.S. patent applicationSer. No. 10/841,192 entitled “Weapon and Weapon System Employing TheSame,” to Roemerman, et al., filed May 7, 2004, U.S. Pat. No. 7,530,315entitled “Weapon and Weapon System Employing the Same,” to Tepera, etal., issued May 5, 2009, and U.S. Pat. No. 8,117,955 entitled “WeaponInterface System and Delivery Platform Employing the Same,” toRoemerman, et al., issued Feb. 21, 2012, which are incorporated hereinby reference. Additionally, a related weapon and weapon system isprovided in U.S. Patent Application Publication No. 2011/0017864entitled “Small Smart Weapon and Weapon System Employing the Same,”published January 27, which is a continuation in part of U.S. Pat. No.7,895,946 entitled “Small Smart Weapon and Weapon System Employing theSame,” issued Mar. 1, 2011, which is a continuation-in-part of U.S. Pat.No. 7,690,304 entitled “Small Smart Weapon and Weapon System Employingthe Same,” issued Apr. 6, 2010, which are incorporated herein byreference. For examples of RFID systems, see U.S. Patent ApplicationPublication No. 2007/0035383, entitled “Radio Frequency IdentificationInterrogation Systems and Methods of Operating the Same,” to Roemerman,et al., published Feb. 15, 2007, U.S. Pat. No. 7,019,650 entitled“Interrogator and Interrogation System Employing the Same,” to Volpi, etal., issued on Mar. 28, 2006, U.S. Pat. No. 7,501,948, entitled“Interrogation System Employing Prior Knowledge About An Object ToDiscern An Identity Thereof,” to Roemerman, et al., issued Mar. 10,2009, U.S. Patent Application Publication No. 2006/0017545, entitled“Radio Frequency Identification Interrogation Systems and Methods ofOperating the Same,” to Volpi, et al., published Jan. 26, 2006, U.S.Patent Application Publication No. 2005/0201450, entitled “InterrogatorAnd Interrogation System Employing The Same,” to Volpi, et al.,published Sep. 15, 2005, and U.S. Pat. No. 8,063,760, entitled“Interrogator and Interrogation System Employing the Same,” to Volpi, etal., issued Nov. 22, 2011, all of which are incorporated herein byreference.

As disclosed herein, synthetic aperture radar (“SAR”) techniques can beapplied to the field of RFID to provide precise tag locations. A movingreader (or interrogator) can collect the reverse link carrier phasetrajectory from RFID tag(s) and then process the same using SAR-liketechniques to create an image of the location of the RFID tag(s).Backscatter modulation from the RFID tag(s) is used to discriminate RFIDtag reflected energy from non-RFID tag reflected energy. As an adjunctinvention, if at least some RFID tag positions are at apriori knownpositions, a reader's position can be ascertained yielding a positiondetermination system applicable in environments where other navigationsystems may not be suitable or denied, as examples global positioningsystem (“GPS”) denial, or indoors.

Also, if the RFID tags (or objects containing the RFID tags) are moving(for example on an automobile, truck or boat) the reader can bestationary and still determine RFID tag positions. A feature foremploying SAR-like techniques to localize the RFID tag positions is thatthere is relative motion between the reader and the RFID tag(s).Multiple readers acting in concert may have advantages in certainsituations. These multistatic systems can exploit diversity techniquesto extend range. Also, the radar cross section (“RCS”) is angledependant and may be enhanced through bistatic operation where thetransmitting reader and the receiving reader are at different locations.

Employing frequency-hopping waveforms (e.g., stepped frequency) can alsobe advantageous, reducing the level of certain SAR processing artifactsand improving resolution. Additionally, the systems herein comprehendthe integration of RFID tags and synthetic aperture radar with othertechnologies for increased accuracy and/or robustness including, but notlimited to, cell tower triangulation and GPS/inertial integratednavigation. Additionally, the RFID system as described hereincomprehends the use of RFID tag identification when only a partialsignature can be decoded.

Referring now to FIG. 1, illustrated is a block diagram of an embodimentof a synthetic aperture radar processor. As mentioned above, a RFIDtag's response may be coherently integrated over the RFID tag's responseinterval by using a local reference equal to the RFID tag's expectedresponse. Multiple RFID tag responses can then be coherently integratedfollowing a corner turning memory (designated “CTM”) working on thepresumption that the RFID tag responses have the same RF phase(equivalent to setting θ_(hypothesis)=0). In the SAR approach, a RFIDtag response phase trajectory is hypothesized based on knowledge of thereader's motion as a function of time, removed from the received signal,and then coherently sum the resultant. If there was indeed a RFID tag atthat location, the response will be strong. The equation below specifiesthe hypothesis using x, y and z coordinates.

${\theta_{hypothesis}(t)} = {\frac{2\pi}{\lambda}\sqrt{\begin{matrix}{\left( {{x(t)}_{reader} - x_{hypothesis}} \right)^{2} + \left( {{y(t)}_{reader} -} \right.} \\{\left. y_{hypothesis} \right)^{2} + \left( {{z(t)}_{reader} - z_{hypothesis}} \right)^{2}}\end{matrix}}}$

More specifically, the process starts with a raw I+jQ sample datacollection process triggered by a scroll identification (“ID”) requesttriggering event. This trigger denotes the expected time at which theRFID tag is expected to begin responding to a read interrogation. Theraw I+jQ sample data (I=Inphase, Q=Quadrature counterpart) is collectedinto a memory 110 with duration nominally the same as the expected RFIDtag response duration. Strong direct current (“DC”) response terms areremoved in module 115 to eliminate bias, for example, by subtracting offthe mean of I+jQ averaged over the response duration.

Single interrogation response complex correlation responses are computedin module 120 using apriori knowledge of the expected RFID tag response(the reference). This can be implemented using time domain processing(e.g., MATLAB “xcorr” function) but may be more efficiently performedusing frequency domain methods employing complex fast Fourier transforms(“FFTs”). This is a common correlation technique well known to thoseskilled in the art and is mathematically analogous to a convolutionoperation. If the incoming signal matches the referenced signal, astrong correlation peak will be generated. It should be noted that themodule 120 may include a filter (such as a mismatched filter describedbelow) to further enhance a resolution of the correlation peak. Timedomain complex correlation responses are written into the corner turningmemory CTM by row with each column corresponding to a specific relativedelay between the reference signal and the recorded signal in memory110. Once the desired number of interrogation responses (“N” in corningturning memory CTM) has been obtained, correlation responses are readout by column and phase corrected (by logic 125) in accordance with theθ_(hypothesis)(t) specific to a particular hypothesized RFID taglocation.

For each hypothesized location, N corrected RFID tag responses arecoherently integrated in a summer 130 and upon completion, envelopedetected in module 135, preferably using a square law detection method(I²+Q²). In module 140, the resultant “z”, associated with a particularRFID tag location hypothesis, is compared to a threshold V_(t)(N) and ifit is greater, the RFID tag is declared present at that location. Oneskilled in the art will recognize several variations in the processingof FIG. 1; specifically, alternative crosscorrelation techniques formodule 120, various noncoherent detection schemes for 150 (e.g.(I²+Q²)^(1/2)) and possibly including the addition of noncoherentintegration steps in module 135. Sequential observer techniques (e.g.,Tong detector or M of N) may also be contemplated for module 140 (see,e.g., “Low-Level GPS/DSSS Signal Acquisition Techniques, Tong Detectorvs. M-of-N Detector,” J. Shima, May 21, 2002). It should be noted thatthe SAR processing may be embodied in a processor with associatedmemory. The processor may include one or more of general-purposecomputers, special-purpose computers, microprocessors, digital signalprocessors (“DSPs”), and processors based on a multi-core processorarchitecture, as non-limiting examples.

The memory may be of any type suitable to the local applicationenvironment, and may be implemented using any suitable volatile ornonvolatile data storage technology such as a semiconductor-based memorydevice, a magnetic memory device and system, an optical memory deviceand system, fixed memory, and removable memory. The programs stored inthe memory may include program instructions that, when executed by anassociated processor, enable the communication element to perform tasksas described herein. Exemplary embodiments of the system, subsystems andmodules as described herein may be implemented, at least in part, bycomputer software executable by processors, by hardware, or combinationsthereof.

Turning now to FIG. 2, illustrated is a graphical representation of theradial range of RFID tags as a function of time. Presume that a readeris flying in a straight east to west line, 50 meters south of a tag at[x,y,z]=[0,0,0]. Then, the radial range as a function of time will be asdepicted in FIG. 2. The radial range for each of the three RFID tags isdistinct based on its location, but is less than 100 meters in theillustrated embodiment. The radial range of the flight trajectory mayvary depending on the application such as a 50 meter half circleobserved trajectory or a 50 meter full circle observed trajectory. Inother embodiments, the flight trajectory can be a straight line in thedirection of the target or a logarithmic trajectory in the direction ofthe target.

Turning now to FIG. 3, illustrated is a graphical representation of asynthetic aperture radar image associated with three RFID tags. Thethree RFID tags are resolved in position based on their expected phasetrajectories. Other forms of SAR such as side looking SAR and spotlightSAR would yield similar images. The reader is operating at 915 megahertz(“MHz) and moving at 50 miles-per-hour (“mph”) at an altitude of 50meters (“m”). The reader is performing 100 interrogations per second andthe delta position is 0.75146 lambda.

Turning now to FIG. 4, illustrated is a graphical representation of aRFID backscatter spectrum. The illustrated embodiment shows the measuredspectrum of a particular auto-identification (“ID”) RFID's backscatterover a ±1 MHz span. Non-modulating passive reflectors (e.g., metal cans)yield a maximum Doppler offset of ±90 hertz (“Hz”) and so can befiltered out with a strong notch filter without losing much of thedesired RFID tag signal.

Turning now to FIG. 5, illustrated is a graphical representation of afrequency stepping waveform with a randomized step size to yield bettertemporal sidelobe performance. The resolution of a SAR image isdependent on the angular extent of the views of the RFID tag. Seeing theRFID tag over a wide angular extent yields much higher resolution, butat the expense of long look times and indirectly, lower transmit antennagain. Not knowing where the RFID tag is, a fairly broad antenna beam isbeneficial with resultant low gain.

Thus, synthetic ranging can sharpen image by lowering the angular extentrequirements and can extend detection range since it allows narrowerbeam transmit antenna halving the field of regard and doubling therange. Non uniform stepping can mitigate grating temporal lobes. Theresults may be improved with a mismatched filter (“MMF”) approach andcenter weight energy distribution. Frequency hopping aspect makes thejamming system harder.

Regarding the filters, matched filters obtain best signal-to-noise ratio(“SNR”) performance in a white Gaussian noise environment, but often atexpense of large temporal sidelobes. In radar applications, largetemporal sidelobes can be interpreted as false targets and incommunications systems, they limit the ability to distinguish variousmultipath components when probing the channel response function. Awaveform design that reduces temporal sidelobe levels is preferable.Binary phase shift keyed (“BPSK”) waveforms combined with mismatchedfilter receivers specifically designed to lower temporal sidelobe levelswould be beneficial.

Turning now to FIGS. 6 and 7, illustrated are matched and mismatchedfilter responses, respectively. A length five Barker code [1,1,1,−1,1]is transmitted using a BPSK signal format. In the matched filterresponse as a function of time, in addition to a large desired response,four large temporal sidelobes at a 20 log (1/5)=−14 decibel (“dB”) levelare present. In the mismatched filter responses, the finite impulseresponse (“FIR”) filter tap values are[0,−1,2,−2,0,5,−8,5,8,5,0,−2,−1,0,0] with integer chip spacing. Now,peak sidelobe levels are at −30 dB, an improvement of 16 dB. RegardingSNR loss; the mismatched filter response only loses 0.6 dB at thecorrelation peak. This is because the mismatched filter design enhances(e.g., maximizes) output SNR, while at the same time reducing (e.g.,minimizes) peak sidelobes; two opposing constraints. Ease ofimplementation is also a consideration in that filter taps were limitedto three bit precision in this example.

Turning now to FIGS. 8 and 9, illustrated are graphical representationsof a synthetic aperture radar image associated with three RFID tags.FIGS. 8 and 9 show resultant SAR images without and with the steppedfrequency waveform of FIG. 5.

Synthetic ranging techniques using stepped frequency waveforms can beemployed to sharpen the image and to mitigate SAR artifacts. In bothcases, the reader is operating at 915 megahertz (“MHz) and moving at 65miles-per-hour (“mph”) at an altitude of 50 meters (“m”). The reader isperforming 200 interrogations per second and the delta position is0.48845 lambda.

Turning now to FIG. 10, illustrated is a block diagram of an embodimentof an active RFID tag. The active RFID tag receives a reference signal(e.g., an S-band reference signal) from, for instance, an unmannedaerial vehicle (“UAV”) at a receiver Rx via a receive antenna RA. Thereference signal provides a coherent reference for the active RFID tagvia a low gain directional antenna (e.g., 3 decibels) and low transmitpower (e.g., 0.25 watts). A transmitter Tx of the active RFID tagresponds by sending a return signal (e.g., a frequency hopping returnsignal in the X-band) via a transmit antenna Tx to the UAV. Exemplarytransmit powers for the active RFID tag for a frequency of 10 gigahertz(“GHz”) are set forth below in Table I.

TABLE I Transmit Reader Transmit Duty Near Pulse RFID Tag Max RangeCycle Factor Rx Gain Sensitivity Gain Tx Power (kilometers) (percent)(decibels) (decibels) (decibels) (decibels) (milliwatts) 6 0.1 5 3 −3 −212.81 12 0.1 5 3 −3 −2 51.23 6 1 5 3 −3 −2 1.28 12 1 5 3 −3 −2 5.12 6 105 3 −3 −2 0.13 12 10 5 3 −3 −2 0.51

Exemplary transmit powers for the active RFID tag for a frequency of 2.7gigahertz (“GHz”) are set forth below in Table II.

TABLE II Transmit Reader Transmit Duty Near Pulse RFID Tag Max RangeCycle Factor Rx Gain Sensitivity Gain Tx Power (kilometers) (percent)(decibels) (decibels) (decibels) (decibels) (milliwatts) 6 0.1 5 3 −3 −20.93 12 0.1 5 3 −3 −2 3.73 6 1 5 3 −3 −2 0.09 12 1 5 3 −3 −2 0.37 6 10 53 −3 −2 0.01 12 10 5 3 −3 −2 0.04

While the active RFID tag includes an active transmitter, thesemi-active RFID tags employ backscatter modulation (no activetransmitter) to provide a return signal. Exemplary characteristics of asemi-active RFID tag are set forth below in Table III.

TABLE III Characteristics X-Band (inches) S-Band (inches) AntennaPrimary Aperture 12 12 Antenna Gain (decibels) 30 18 Transmit Power(watts at 10 20 20 percent Tx duty cycle) Swath Width (kilometers) 2 to5 km offset 3.5 to 5 km offset

The following Table IV provides a comparison of active and semi-activeRFID tags.

TABLE IV Semi-active RFID Tags Active RFID Tags UAV Size, weight andpower Modest size, weight and Features requirements limit UAV powercandidates More UAV candidates 12 inch antenna 20 watt transmitter at 10percent duty cycle UAV Impaired because of high Reasonably covertCovertness effective radiated power (“ERP”) Easy to detect interest(keep antenna directed to the target) RFID Tag Less complex designHigher power consumption Sufficient size to obtain and unit cost higherRCS RFID tag is detectable S-Band RFID tag is physically duringoperation larger but coding assists RFID tag locally detectable CoverageX-band 2 km swaths at 5 km 10 to 20 km wide swaths standoff rangecentered on UAV S-band 3.5 km swaths at 5 km 12 km range standoff rangeInterleave scan to increase area search rate

Alternative embodiments comprehend the use of either passive,semi-active or active RFID tags. The passive RFID tags do not have abattery and rely on the incident signal to power the return signaltherefrom. The active RFID tags have the advantage of further range butat the expense of having to have a battery and usually, greater cost.The active RFID tag generates a reverse link signal coherent with thereader's interrogation signal. The interrogator may phaselock to theincoming signal to generate a local reference and mix to anotherfrequency for the reverse link.

The system as described herein accommodates low resolution search andhigh resolution track modes using SAR processing and dynamic flight pathmodification. The SAR processor may be located on a vehicle orelsewhere. The system can process backhaul raw analog-to-digital (“ADC”)samples, preprocess and backhaul to a ground station, store mission datain memory for non real-time systems and the addition of a GPS receiveror transdigitizer on the RFID tag or as part of the system.

The reader (interrogator) as described herein can determine, if the RFIDtags are at known positions, by its own position and velocity as itmoves about by using SAR techniques combined with a triangulationprocess. By hypothesizing its own trajectory (x_(READER)(t),y_(READER)(t), z_(READER)(t)), the reader can arrive at a set of threeor more phase trajectories according to the equations below and thentest to see if they match responses from the RFID tags. If all of thephase trajectories common to the hypothesized reader trajectory yield astrong response, then the reader's location is identified. Once thetrack is established, a much smaller set of reader trajectoryhypothesizes can be used.

${\frac{2\pi}{\lambda}\sqrt{\left( {x_{READER} - x_{1}} \right)^{2} + \left( {y_{READER} - y_{1}} \right)^{2} + \left( {z_{READER} - z_{1}} \right)^{2}}} = \theta_{1}$${\frac{2\pi}{\lambda}\sqrt{\left( {x_{READER} - x_{2}} \right)^{2} + \left( {y_{READER} - y_{2}} \right)^{2} + \left( {z_{READER} - z_{2}} \right)^{2}}} = \theta_{2}$${\frac{2\pi}{\lambda}\sqrt{\left( {x_{READER} - x_{3}} \right)^{2} + \left( {y_{READER} - y_{3}} \right)^{2} + \left( {z_{READER} - z_{3}} \right)^{2}}} = \theta_{3}$

An alternative embodiment includes those applications where the RFIDtags may be moving while the reader remains stationary. SAR techniquesare again used to determine the RFID tag's location by using a set ofhypothesized RFID tag trajectories and seeing which matches to produce astrong response. Such an approach will determine whether the RFID tag isentering or leaving a specific location. In an alternative embodiment,it is used in automated sorting equipment to decide when to open andclose gates to direct the RFID tag (and the tagged object) to aparticular destination.

In yet another embodiment, multistatic techniques are employed toenhance RCS, provide diversity channels, and enhance RFID taglocalization by providing multiple look angles. Using a time andfrequency source (e.g., GPS), interrogation transmitters and RFID tagresponse receivers can be coordinated so as to permit SAR processingwith the transmitter and receiver at different locations. This bistaticmode has several advantages. It eases isolation problems associated withtransmitting and receiving at the same frequency simultaneously, whichis a particular problem for backscatter signal reception. Bistatic RCSis often much larger than monostatic RCS, particularly when the incidentand reflection angles are similar. Having multiple receivers placed atseparate locations provides diversity reception channels, therebyenhancing RFID tag signal reception.

In some applications, using the full set of SAR processing techniques isneither necessary nor practical. If the objective is merely to point tothe RFID tag in order to find it, the following observations can bemade. Moving towards the RFID tag yields advancing phase, moving awayretards phase and, perpendicular motion yields no phase change. Withthese three hypotheses and a processing approach as described withrespect to FIG. 1, the RFID system may be embodied in a handheld orweapons based interrogator that indicates which hypothesis fits best,namely, moving towards, away or perpendicular.

In yet another embodiment, an inertial measurement unit (“IMU”) assistedreader located on the vehicle could read RFID tags as they pass by,perform SAR processing and then based on the RFID tag's identity and adatabase, make an assessment of position. Passive RFID tags could beembedded in the pavement every few feet, possibly as part of a paintstriping operation. Then, a survey vehicle equipped with high endGPS/inertial technology (such as provided by Applanix) would map out theRFID tags and create a database that could be downloaded to vehicles orqueried via radio. The advantage of such a system is that it could beprecise (˜5 centimeters (“cm”)) and it would work in environments whereGPS does not work such as in tunnels, parking structures, underpasses,etc., or even in GPS denied environments. It could be used to providelane information as well that might be used in providing laneassignments. It would also be useful in accident reconstruction. Inanother embodiment, the UAV configuration would also include aninertial/SAR to map out tag locations and then convey the information tothe weapon.

In yet another embodiment, readers are placed at known stationarylocations to follow location of a vehicle using SAR techniques. The SARaspect yields area coverage as opposed to “pass by a reader” coverage.This embodiment is useful in monitoring traffic and parking space usage.In an exemplary application, a ticket containing an RFID tag would allowthe parking system operator to create a dynamic database showing wherethe vehicle is actually parked. This would be useful in assessingavailability, finding lost vehicles, fraud detection and enforcingvariable rate structures (e.g., certain parking locations cost more).This variant, when combined with a low cost RF transponder, provideslocation information to the vehicle or pedestrian carrying theappropriate RFID tag. Such a system could also be used to monitorprisoner locations, shopping cart trolley locations or pallet locationsin near real-time. Active and semi active variants are also possible toprovide extended range. Yet another modification is to place an RFID tagon a vehicle or item that needs to be surreptiously tracked.

In yet another embodiment, RFID tags with known signatures are placed atsurveyed locations. When the RFID tags are detected by the SARprocessing, their apriori known locations are used in a dimensional mapsimilar to that shown in FIGS. 3 and 6 along with a detected RFID tagwhose position is not known. Using differential interpolating navigationtechniques, the position of the unknown RFID tag can be determined withrespect to the positions of the known RFID tags.

A synthetic aperture radar (“SAR”) is a coherent mostly airborne orspaceborne often sidelooking radar system that utilizes the flight pathof the platform to simulate an extremely large antenna or apertureelectronically. Due to this long derived or ‘synthetic’ aperture,high-resolution remote sensing imagery is capable of being generated.Over time, individual transmit/receive cycles (e.g., in the form ofpulse repetition time (“PRT”) between subsequent pulses) are completedwith the data from each cycle being stored electronically. The signalprocessing uses magnitude and phase of the received signals oversuccessive pulses from elements of a synthetic aperture. After a givennumber of cycles, the stored data is recombined (taking into account theDoppler effects inherent in the different transmitter to target geometryin each succeeding cycle) to create a high resolution image of theterrain being over flown. FIG. 9 illustrates a view of an embodiment ofa radar silhouette of a ship produced with an inverse synthetic apertureradar (“ISAR”) processor. Inverse synthetic aperture radar is simply thesame technique whereby the object, rather than the radar is moving.However, by the principle of reciprocity, the basic physics is the same.

Turning now to FIG. 12, illustrated is a view of an embodiment of asynthetic length of a derived synthetic aperture radar antenna.Synthetic aperture radar works similar to a phased array, but contraryto the large number of parallel antenna elements of which a phased arrayis typically composed, SAR uses one antenna in time-multiplex. Thedifferent geometric positions of the antenna elements are the result ofthe moving platform as shown in FIG. 10.

A SAR processor stores the radar returned signals, as amplitudes andphases, for the time period T from position A to D. Now it is possibleto reconstruct the signal which would have been obtained by an antennaof length v*T, where v is the platform speed. As the line of sightdirection changes along the radar platform trajectory, a syntheticaperture is produced by signal processing that has the effect oflengthening the antenna. Making the time period T large makes the“synthetic aperture” large and hence a higher resolution can beachieved.

As a target (e.g., a ship) first enters the radar beam, thebackscattered echoes from each transmitted pulse begin to be recorded.As the platform continues to move forward, the echoes from the targetfor each pulse are recorded during the time that the target is withinthe beam. The point at which the target leaves the view of the radarbeam some time later, determines the length of the simulated orsynthesized antenna. The synthesized expanding beamwidth, combined withthe increased time a target is within the beam as ground rangeincreases, balance each other, such that the resolution remains constantacross the swath. The achievable azimuth resolution of a SAR isapproximately equal to one-half the length of the actual (real) antennaand does not depend on platform altitude (distance).

Some exemplary features of synthetic aperture radar include a stable,fully coherent transmitter, an efficient and powerful SAR-processor andknowledge of the flight path and the velocity of the platform. Usingthis approach, systems have been constructed that achieve resolutionsfrom airborne platforms that would be impractical, if not impossibleusing conventional radar processing.

Synthetic aperture radar is partnered by what is termed inversesynthetic aperture radar (again, ISAR) technology which in the broadestterms, utilizes the movement of the target rather than the emitter tocreate the synthetic aperture. Inverse synthetic aperture radars havebeen used aboard maritime patrol aircraft to provide radar images ofsufficient quality to allow them to be used for target recognitionpurposes.

Turning now to FIG. 13, illustrated is a view of an embodiment ofsynthetic aperture radar processing. In the illustrated embodiment, eachpulse repetition time (“PRT”) between subsequent pulses oftransmit/receive cycles moves across a shift register as the SARreceiver moves. These snapshot radar images are then combined to form aSAR image. Accurate time and position is employed as the snapshots arecombined coherently in phase and magnitude just as if the snapshots hadbeen simultaneously impinged on a large multi-element array. For abetter understanding of synthetic aperture radar and related processing,see “Synthetic Aperture Radar: Systems and Signal Processing,” John C.Curlander, Wiley Series in Remote Sensing and Image Processing, November1991 and “Digital Processing of Synthetic Aperture Radar Data:Algorithms and Implementation,” Ian G. Cummings, Artech House RemoteSensing Library, January 2005, both of which are incorporated byreference.

Turning now to FIGS. 14 and 15, illustrated are views of embodiments ofvehicles (e.g., UAVs) including a reader (designated “RDR”, orinterrogator). In one embodiment, acquisition begins with a wide areasearch from a UAV. In other embodiments, acquisition can be incorporatedusing a number of theater and national assets including ground,airborne, and space based assets as well as the delivery vehicle orweapon itself.

The RFID system of FIG. 14 searches a wide swath with the searchingantenna of the reader RDR on the side of the vehicle similar to a sidelooking SAR. This is beneficial if the position of the target (RFID tag)is not known apriori. Once the target (RFID tag) is identified, itsposition can be transferred to an onboard weapon (delivery vehicle), forexample, a guided bomb. Alternatively, the receiver can be containeddirectly within the delivery vehicle.

In FIG. 15, the reader RDR is located on a guided bomb attached to thevehicle. The reader RDR can assist the guidance section to guide thebomb to a target (RFID tagged target). In a variation of thisembodiment, the reader can be located within the weapon and the SARprocessor can be in the vehicle (e.g., UAV). Thus, full SAR processingis possible prior to launch. However, post launch, the reader can stillbe used to home onto the target. This also has an added advantage ofreducing weapon cost as the SAR processor is reusable. Yet anothervariant is to have the SAR processor remote from the weapon andavailable via a communications data link to the weapon.

The platform (e.g., UAV) acquires the RFID tagged target using SAR likesignal processing and results in a three dimensional fix on the target.The position fix is accurate enough for guidance, even with a smallwarhead. Depending on launch platform, weapon(s) and size(s), the RFIDsystem can be partitioned between the two vehicles. Any UAV may beemployed with the RFID system including, without limitation, a Predator,Pioneer, Hunter, Global Hawk, Shadow 200, Fire Scout and Dragon Eye. TheUAVs may include a tactical common data link to communicate informationfrom the reader to another location. The tactical common data link mayoperate in the KU band with 200 kilo-bit-per-second forward command linkand 10 mega-bit-per-second reverse link having a range of 150 nauticalmiles.

Turning now to FIGS. 16 and 17, illustrated are views of embodiments ofa vehicle (e.g., UAVs) and a bomb, respectively, including a reader(designated “RDR”, or interrogator) separated into multiple sections. Asshown in FIGS. 16 and 17, an antenna (designated “ANT”) of the readerRDR is located on the nose of the UAV and bomb, respectively. In FIG.14, the reader RDR is further bifurcated into two subsystems.Additionally, a SAR processor of the reader RDR may be located remotefrom the UAV and communicate therewith via a tactical common data link.The antennas may be slip-on sleeve antennas for kit weapons. Theterminal guidance are weapon and mission dependent and include pureinertial guidance to last known point, use of a forward aperture and/ordeviated flight path and active verses semi active seeking.

Exemplary transmit powers and search rates for a UAV at a 50 meteraltitude are set forth below in Table V.

TABLE V Reference Data Range (feet) 29.50 29.50 29.50 29.50 Range(meters) 8.99 8.99 8.99 8.99 Tx Power (watts) 0.10 0.10 0.10 0.10Transmit Power Propagation 2.00 2.00 2.00 2.00 Exponent Desired Range100.00 150.00 200.00 250.00 (meters) Increase in Tx 123.69 278.30 494.75773.05 Power (time ref.) Transmit Power 12.37 27.83 49.48 77.30 (watts)Swath Width UAV Altitude 50.00 50.00 50.00 50.00 (meters) Slant Range100.00 150.00 200.00 250.00 (meters) One Sided Swath 86.60 141.42 193.65244.95 Width (metes) Two Sided 173.21 282.84 387.30 489.90 Swath Width(metes) Two Sided 0.11 0.18 0.24 0.30 Swath Width (miles) Search SpeedForward Speed 65.00 65.00 65.00 65.00 (mph) Search Rate 7.00 11.42 15.6419.79 (square mph)Of course, the transmit power will vary depending on the factors (suchas a change in altitude) associated with a particular UAV. It shouldalso be noted that any vehicle may be employed with the system herein.

Thus, SAR-like processing can assist with mapping out unexplodedordinance or any other tagged objects. The resolution depends onobserver trajectory and interrogation rates. The trajectory patterns canbe dynamically optimized. The interrogation patterns can be dynamicallyoptimized based on SNR, number of RFID tags and beam steeringcapabilities. The processing techniques are tolerant of noise via jointSAR/detection techniques that yield improved detection performance andreal time kinematic GPS processing to yield position to about twocentimeters.

Turning now to FIG. 18, illustrated is a graphical representation of asynthetic aperture radar image associated with three RFID tags. ThisFIGURE shows an X-band system with 800 MHz frequency hop spread. Thethree RFID tags are all detected at four meters apart. The processingscheme was a based on a five second/300 meter aperture at −3 dB SNR. Thedetection is tens of decibels above the clutter, wherein the systemdiscriminates each RFID tag from the other, and from the background. Thereader is operating at 10,000 megahertz (“MHz) and moving at 120miles-per-hour (“mph”) at an altitude of 3048 meters (“m”). The readeris performing 100 interrogations per second and the delta position is19.7104 lambda.

Turning now to FIG. 19, illustrated is a graphical representation of asynthetic aperture radar image associated with three RFID tags. ThisFIGURE shows an S-band system. As expected with the longer wavelength,resolution is not as good as X-band system, but is acceptable to delivereven small warheads. In this case the system used a 200 MHz frequencyhop spread at 2700 MHz. The reader is operating at 2700 megahertz (“MHz)and moving at 120 miles-per-hour (“mph”) at an altitude of 3048 meters(“m”). The reader is performing 100 interrogations per second and thedelta position is 5.3218 lambda.

Turning now to FIG. 20, illustrated is a graphical representation of asynthetic aperture radar image associated with three RFID tags. ThisFIGURE shows the results of an X-band system associated with an initialwide area acquisition mode for quickly searching large portions ofpotentially interesting areas. This approach is very efficient atlooking at large areas and can be used in an initial wide areaacquisition mode. This mode provides potential for a wide areaacquisition mode, supporting larger swaths of search area. Subsequent tothis, a narrower acquisition mode may be chosen, if necessary, based onthe information obtained from this initial search. The reader isoperating at 10,000 megahertz (“MHz) and moving at 120 miles-per-hour(“mph”) at an altitude of 3048 meters (“m”). The reader is performing100 interrogations per second and the delta position is 19.7104 lambda.

During initial target acquisition, there may be considerable uncertaintywith regards to target position. This may necessitate large processingresources and or use of lower resolution waveforms/lower resolutionwaveform processing approaches. Once the target(s) are found, the systemcan enter a track mode wherein the reduced uncertainty in targetlocation is used advantageously. This can be in the form of reducedprocessing requirements and/or through the use of a higher resolutionwaveform. For weapons guidance, approximate target coordinates may behanded off to the weapon which performs SAR image formation using areduced uncertainty and possibly with an enhanced resolutionwaveform/waveform processing approach.

Alternatively, the target acquisition system, optionally located on aUAV, may operate in a track/acquisition mode wherein it may tracktarget(s) using enhanced resolution while simultaneously engaged insearch and acquisition process(es). Here simultaneous may imply timeinterleaved operation wherein waveforms switch modes depending onwhether the system is in acquisition or track sub modes. The system mayemploy an interleaved operation in which target(s) are tracked whilstalso looking for others.

Any information with regards to target location may be used to reduceprocessing requirements. If, for instance, the target is known to be onthe ground or at a particular altitude above ground level (e.g., thirdfloor), then a map containing elevation location can be used to reducethe range of possible target locations and thus limit processingrequirements. A digital topographic map would be one example of such amap but other possibilities exist such as a synthetic aperture radargenerated map. Cuing from optical or radar systems tracking the targetmay also be of utility to reduce the possible targetlocations/velocities window. As an example, general moving targetindicator (“GMTI”) radar can cue the SAR-like processing techniques ascould an optical range/bearing system.

The radar responsive RFID tag may cooperate with the interrogator byseeking to enhance its radar cross section (“RCS”) in the direction ofthe interrogator(s). For monostatic modes with a passive or semi-activetag, one such technique would be to employ an array of antenna elementsin the RFID tag, measure relative carrier phase upon reception, andadjust backscatter phases of individual elements so as to enhance theRCS in the direction of the interrogator. For active RFID tags, whichtransmit, a similar concept could be employed wherein relative phases oftransmission of individual elements could be adjusted to enhance thegain back towards the interrogator. In an extension of the concept,instead of adjusting phases, time delays may be adjusted to providebetter wide bandwidth capabilities.

For bistatic modes, wherein emanations from the RFID tag are notreceived at the same location as from which they are received; the RFIDtag may be informed of direction offsets and seek to enhance theRCS/gain in the direction of the intended receiver(s). In oneembodiment, the interrogator transmitting to the RFID tag could conveyoffset information to the RFID tag via data aspects in its transmissionwaveform. The RFID tag upon demodulating this information could adjustphases accordingly.

Under normal circumstances, SAR-like image formation benefits from goodlocation and inertial measurement unit data. Lesser quality data may beacceptable by using multiple RFID tags and/or interrogators to createSAR-like images and so determine the locations of the RFID tag(s). Thismay in fact permit operation in GPS denied environments. This isbecause, if there are multiple RFID tag(s) and/or interrogators at knownrelative locations, additional unknown variables may be solved for. Asan example; if one RFID tag is placed on the target and another isplaced at a known offset with respect to the first, the weapon can solvefor its own relative position. One possible method would be to employ anarray of hypothetical “own trajectories” and determine which one permitssimultaneous good SAR-like focus on both RFID tags. Extending theconcept, additional RFID tags at known relative locations could be usedto improve the relative position/relative trajectory solution.

In some cases, all RFID tags may be offset with respect to the weaponaim point. As an example, one or more RFID tags may be placed at knownoffsets with respect to the weapon aim point. This obviates the need togain physical access to the aim point, and it permits for relativeposition strike options (e.g., close air support). An additional benefitof the above approach is that SAR-like image resolution in the azimuthaldirection is dependent on relative trajectory between the interrogatorand the RFID tag. If the interrogator proceeds directly to the RFID tag,then the synthetic aperture is small and poor azimuthal resolution isthe result. With multiple RFID tags and/or interrogators at dispersedlocations, there is improved likelihood of obtaining good SAR-likeazimuthal resolution even though some pairings offer poor resolution.

Turning now to FIG. 21, illustrated is a view of an embodiment of asystem to locate RFID tags with synthetic aperture radar processing. Thesystem integrates UAV operations with SAR processing to locate the RFIDtags. In one embodiment, the RFID tags are attached to unexplodedordinance (“UXO”) and the UAV is flying in a low and slow trajectory tolocate and map the unexploded ordinance. Other variations such aslocating missing personnel and locating lost items in a warehouse (e.g.,a ground mobile version).

The system uniquely locates passive, semi-active or active RFID tagsusing the signals returned from the RFID tag(s) to be processed bySAR-like processing techniques in space. The use of multiple differenttypes of tags is comprehended. The system has the ability to resolvesimultaneous tags, which lowers the dependence on orthogonal codingresolution techniques. This results in a much greater number of uniquecodes that can be used.

The system as described herein recognizes that there are multiplexingtradeoffs associated therewith. In an embodiment, an upper limit auto-IDinterrogation rate is about 500 interrogations-per-second (“IPS”) withone millisecond (“msec”) interrogation followed by 0.627 msec tagresponse. The SAR image resolution is based on obtaining multipleperspectives. The interrogation rate is a function of vehicle speed andthe SNR. The system can use lower interrogation rates to save power (usesmaller vehicle), to do beam steering to raise the equivalentisotropically radiated power (“EIRP”) on hard to trigger tags and topartition tag responses and mitigate near/far problems. The lowerinterrogation rates and/or tag response SNR tend to lower the resolutionof the SAR image. The parameters can be selected to meet therequirements of a particular application.

Turning now to FIG. 22, illustrated is a view of an embodiment of a RFIDsystem. The RFID system includes a reader or interrogator (a RFIDcorrelating/SAR processing transceiver) RDR and a RFID tag (designated“TG”). The reader RDR is typically vehicle mounted (airborne or ground)and moving to achieve SAR capability. The RFID tag TG is in a fixedlocation or attached to a moving object. Thus, the reader RDR isconfigured to detect the RFID tag TG either statically mounted or on amoving vehicle as well. The correlation of a return signal from the RFIDtag TG at the reader RDR achieves greater detect sensitivity thereof.The RFID tag TG can be passive, semi-active or active. During and afterRFID tag TG detection, SAR processing of the return signal(s) allows thereader RDR to determine a location thereof. This consists of time andvehicle location tagging of the detected return signal(s), bothindividually and collectively.

Turning now to FIG. 23, illustrated is a view of an embodiment of a RFIDsystem. The RFID system includes first and second readers orinterrogators (RFID correlating/SAR processing transceivers) RDR1, RDR2and a RFID tag (designated “TG”). The readers RDR1, RDR2 are typicallyvehicle mounted (airborne or ground) and moving to achieve SARcapability. The RFID tag TG is in a fixed location or attached to amoving object. The readers RDR1, RDR2 work cooperatively wherein a firstreader RDR1 activates the RFID tag TG and both readers RDR1, RDR2receive the return signal(s) from the RFID tag TG. The information isthen combined to improve the reliability of the location of the RFID tagTG.

As above, the readers RDR1, RDR2 are configured to detect the RFID tagTG either statically mounted or on a moving vehicle as well. Thecorrelation of a return signal from the RFID tag TG at the readers RDR1,RDR2 achieves greater detect sensitivity thereof. The RFID tag TG can bepassive, semi-active or active. During and after RFID tag TG detection,SAR processing of the return signal(s) allows the readers RDR1, RDR2 todetermine the location thereof. This consists of time and vehiclelocation tagging of the detected return signal(s), both individually andcollectively.

Turning now to FIG. 24, illustrated is a view of an embodiment of areader or interrogator (a RFID correlating/SAR processing transceiver).The reader includes a controller CT configured to provide overall eventscheduling, timing and directed processing. A transmitter Tx isconfigured to send a coded (modulated) signal (an interrogation signal)consistent with a RFID tag being deployed at the transmitting frequencythrough a diplexer DP and onto an antenna ANT where it is transmittedthrough space. The antenna ANT is a single element design with a welldefined and stable azimuth and elevation beamwidth to produce signalssuitable for SAR processing.

A receiver such as a correlating receiver CR controlled by thecontroller CT performs a correlation function on a return signal fromthe RFID tag and based on the type of RFID tag deployed, may be either ahomodyne or a heterodyne receiver. If the RFID tag is passive, orsemi-active, the correlating receiver CR is typically homodyne. If theRFID tag is active, the correlating receiver CR may be either homodyneor heterodyne. The output of the correlating receiver CR is a digitalsignal sent to a processor such as a SAR processor SAR.

The SAR processor SAR accepts inputs from the correlating receiver CR, asensing module such as a position, velocity, time sensing module PVT andthe controller CT. As discussed previously, the SAR processor SARprocesses the data from the correlating receiver and the antenna ANT asthough it were a continuous antenna of very considerable length that isdefined by the vehicles path in space that contains the reader.

The position, velocity, time sensing module PVT obtains and outputsposition of the reader, velocity of the reader, and time data(associated with the return signal from the RFID tag) of sufficientaccuracy to the SAR processor SAR. Using the information from theposition, velocity, time sensing module PVT and the information from thecorrelating receiver CR, a “synthetic aperture array” antenna isconstructed with the property of enhanced resolution to locate aposition of the RFID tag that would be achieved were this array toactually exist in physical form. Thus, the SAR processor SAR employssynthetic aperture radar processing on the aforementioned information(including the return signal via the correlating receiver CR) to locatea position of the RFID tag. A common embodiment of this function is touse a NAVSTAR GPS receiver. Other embodiments such as atomic clocks andhighly accurate inertial systems are also possible including integratedsystems consisting of GPS and inertial systems integrated to provide asingle solution. It should be noted that the correlating receiver CRand/or the SAR processor SAR may include a filter (such as themismatched filter described above) to further enhance a resolution ofthe position of the RFID tag.

A data logger DL accepts the time RFID tagged data from the SARprocessor SAR, and the position, velocity, time sensing module PVT andis controlled by the controller CT. The data logger DL logs all relevantdata for any post processing and analysis. Alternatively, or inaddition, a data link may be provided via an antenna ANT, which may be areal-time link or batched as necessary. As an alternative, the SARprocessing may be external to the vehicle carrying the reader. In thisinstance, the time and position tagged RFID data is data linked for SARprocessing at a remote site. The results could then be returned to thevehicle via the data link. In another embodiment, a data recorder DRrecords the time tagged RFID data for SAR processing at a later time.

The transmit and receive frequencies chosen may be determined by, butare not limited to, any frequencies currently in use with RFID tags.This RFID system comprehends the use of other frequencies chosenspecifically to optimize the system's performance. This RFID system alsoclaims the benefits of apriori RFID tag knowledge wherein identifyingcertain RFID tags also identifies the object attached thereto. The RFIDsystem claims the use of unique identifying codes including, but notlimited to, orthogonal codes (e.g., Walsh codes or Kasami sequences) toimprove resolution and RFID tag discrimination. The RFID system may useencrypted codes or other processing techniques such as, but not limitedto, code division multiple access (“CDMA”) or spread spectrum techniquesto limit or avoid detection.

The RFID system may also employ multiple RFID correlating/SAR processorsoperating in a cooperative manner to achieve greater accuracy of theRFID tag's location. In this embodiment, the data may be combined toreduce or eliminate portions of the data missing or having low quality.Also, post processing or the benefit of a data link sufficient forcooperation is employed. Multiple readers acting in concert may haveadvantages in certain situations. These multistatic systems can exploitdiversity techniques to extend range. Also, radar cross section (“RCS”)is angle dependant and may be enhanced through bistatic operation wherethe transmitting reader and the receiving reader are at differentlocations.

The RFID system may employ multiple tags and the system has thecapability to query a single RFID tag and, in that instance, all otherRFID tags remain silent, whether active or not. Of course, when multipletags are present, ones of or all of the multiple tags may respond. Inthis manner, the vehicle can determine the location of multiple tagssimultaneously. Applications include, but are not limited to,discrimination of RFID tags according to being friendly or hostile, andrelative locations with respect to each other. In yet anotherembodiment, multiple tags at known locations may provide geodeticinformation to a vehicle in those instances when GPS is denied. In suchan instance, the RFID tags locations, once determined, can be used toderive the vehicle location via standard triangulation techniques.

The RFID system may operate in different operational modes. In a RFIDtag active mode, the RFID tag enters a mode whereby it will respond ifproperly queried. In an acquisition mode, the RFID tag initiallyresponds to the reader and, in a data taking mode, the RFID tag providessufficient data for SAR processing to locate the position of the RFIDtag. In a track mode, the signal processing continuously tracks the RFIDtag. This could occur during a weapon launch as a result of thedetection of the location of the RFID tag and the weapon homes onto theRFID tags return signal(s).

Also, if the RFID tag(s) are moving (e.g., on an automobile, truck, orboat) the reader can be stationary and still determine RFID tagpositions. The relative motion between the reader and RFID tag(s) allowsthe SAR-like processing techniques to localize the position of the RFIDtag(s).

Also, frequency-hopping waveforms (e.g., stepped frequency) can also beadvantageously employed, reducing the level of certain SAR processingartifacts and improving resolution. The RFID system may integrate SARprocessing with other technologies (e.g., cell tower triangulation andGPS/inertial integrated navigation) for increased accuracy and/orrobustness. Additionally, the RFID tag may be identified when only apartial signature can be decoded.

Turning now to FIGS. 25 and 26, illustrated is a view of an embodimentof a RFID tag. The RFID tag includes an antenna ANT, electronics EL, abattery BAT and a mount MT. The electronics EL include a communicationelement (e.g., receiver, transmitter and/or transceiver) and aprocessing function. The battery BAT may be connected to an externalpower source. The mount provides any material or construction (e.g.,adhesives, mechanical mounts and/or magnets) to mount the RFID tag ontoa desired object to be tracked. While the illustrated embodimentprovides an active RFID tag operating at 2.4 gigahertz, passive,semi-active, and active tags operating at other frequencies may be usedas well. Of course, any operating frequency may be employed with theRFID system. Exemplary dimensions of the RFID tag are 16 mm×12 mm×12 mm.As illustrated in FIG. 26, the antenna ANT employs two linear fractalantennas configured to provide a circularly polarized signal (90 degreephase and delay) plus a matching network on a ceramic substrate.Exemplary dimensions for the elements of the antenna ANT are illustratedon the FIGURE. It should be noted that the RFID tag is constructed inlayers to accommodate a compact design.

This RFID system may include standard RFID tags or custom RFID tagsdeveloped for use with correlating/SAR processing. Typically an encodedsignal from the reader will initiate the function whereby the RFID tagwill respond in the event the encoded signal is correct. The RFID tagmay become active or able to respond to a query based on a predeterminedtime, or when an activating signal (interrogation signal) is firsttransmitted or at some time after the activating signal is transmitted.The activating signal can be unique and need not be part of the RFIDstandard interface specification. The RFID tag may also come alive atpredetermined times for a period of time to see if a reader is presentand then return to an inactive mode if no reader is present.

The RFID tag may become active due to the occurrence of any eventincluding non-electromagnetic events. These include, but are not limitedto, events such as movement, incident light, sound, temperature,presence of a non-related electromagnetic signal, or related activatingsignal, or any relevant external activity indicating the RFID tag shouldbecome active. Of course, the RFID system may integrate sensors toimplement these functions. If a passive RFID tag is used, the incidenttransmitted energy should be sufficient to charge the included capacitorproviding the energy to activate and operate the RFID tag.

If a semi-active tag is used, an included battery provides the energynecessary to operate the RFID tag. However, it does not operate as anactive transmitter, but can only reflect, in varying degrees (i.e.,modulate) the energy from the reader. If an active RFID tag is used, theRFID tag performs in a manner similar to a fully active transceiver inthat it possesses an active transmitter and is capable of responding(i.e., transmitting) to a query from the reader on the vehicle using adifferent standard, including different frequencies. The power source ofthe RFID tag for semi-active and active operation may be from anincluded power source including, but not limited to, a battery, fuelcell, solar cell or the like and may also be able to derive power from asource not directly a part of the RFID tag (e.g., external battery, orother external power source).

It should be understood that different communication techniques such as,but not limited to, spread spectrum techniques may be employed with theRFID system. This could increase system range capability and alsocontribute to system stealthness by reducing signal detectability. Asmentioned above, the RFID tags may be customized whereby elementsincluding, but not limited to, antenna polarization, antenna typeincluding fractal antennas, level of integration, unique means of tagplacement, power source, and the like may be employed to advantage.

The RFID tag may operate in different modes of operation. In an inertmode, the RFID tag is not responding to any command other than thecommand to become active. This command can be internally or externallygenerated. In a listen mode, the RFID tag is active, but does nottransmit or respond to any signals unless properly interrogated by thereader. Related modes to the listen mode include respond once or respondmany (i.e., once queried continue to transmit until directed otherwise).In a ping mode, at predetermined (fixed or random) intervals, the RFIDtag spontaneously transmits a signal to alert any reader that may be inthe area without the reader first having to initiate a query command. Ina standard mode, the RFID tag responds every time a legitimate commandto do so is received from the reader. In a homing mode, the RFID tagresponds with a high amplitude return signal when so directed. Anapplication for this might be to aid in homing a weapon onto the RFIDtag in the last few seconds before impact. In a self destruct mode, theRFID tag is irretrievably destroyed. Related modes to the self destructmode include destruction upon an external command, at a predeterminedtime or at a future time downloaded thereto by the reader.

Thus, an interrogator and system employing the same have been introducedherein. In one embodiment, the interrogator includes a receiver (e.g., acorrelating receiver) configured to receive a return signal from a tag(e.g., a RFID tag being active, semi-active or passive) and a sensingmodule (e.g., a position, velocity and time sensing module) configuredto provide a time associated with the return signal. The interrogatoralso includes a processor (e.g., a synthetic aperture radar processor)configured to employ synthetic aperture radar processing on the returnsignal in accordance with the time to locate a position of the tag. Theinterrogator also includes a transmitter configured to code aninterrogation signal for the tag and an antenna configured to send theinterrogation signal to the tag. The interrogator also includes acontroller configured to control an operation of the transmitter, thereceiver, the sensing module and the processor. The sensing module ofthe interrogator is configured to provide a position of the interrogatorand a velocity of the interrogator and the processor is configured toemploy synthetic aperture radar processing on the return signal inaccordance with the position of the interrogator, the velocity of theinterrogator and the time to locate the position of the tag. A datalogger of the interrogator is configured to log the position of theinterrogator, the velocity of the interrogator, the time and theposition of the tag. Also, the tag is configured to move relative to theinterrogator.

Additionally, exemplary embodiments of the present invention have beenillustrated with reference to specific components. Those skilled in theart are aware, however, that components may be substituted (notnecessarily with components of the same type) to create desiredconditions or accomplish desired results. For instance, multiplecomponents may be substituted for a single component and vice-versa. Theprinciples of the present invention may be applied to a wide variety ofweapon systems. Those skilled in the art will recognize that otherembodiments of the invention can be incorporated into a system employingdevices capable of responding to excitation of energy impinging on them,as an example RF tags of various types, and detection approachesincluding, but not limited to, synthetic aperture radar techniques.

As described above, the exemplary embodiments provide both a method andcorresponding apparatus consisting of various modules providingfunctionality for performing the steps of the method. The modules may beimplemented as hardware (including an integrated circuit such as anapplication specific integrated circuit), or may be implemented assoftware or firmware for execution by a computer processor. Inparticular, in the case of firmware or software, the exemplaryembodiment can be provided as a computer program product including acomputer readable storage structure or medium embodying computer programcode (i.e., software or firmware) thereon for execution by the computerprocessor.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form. Moreover, the scope ofthe present application is not intended to be limited to the particularembodiments of the process, machine, manufacture, composition of matter,means, methods and steps described in the specification. As one ofordinary skill in the art will readily appreciate from the disclosure ofthe present invention, processes, machines, manufacture, compositions ofmatter, means, methods, or steps, presently existing or later to bedeveloped, that perform substantially the same function or achievesubstantially the same result as the corresponding embodiments describedherein may be utilized according to the present invention. Accordingly,the appended claims are intended to include within their scope suchprocesses, machines, manufacture, compositions of matter, means,methods, or steps.

What is claimed is:
 1. An interrogator, comprising: a receiverconfigured to receive return signals from a tag; a sensing moduleconfigured to provide time estimates associated with said returnsignals; and a processor configured to employ synthetic aperture radarprocessing on said return signals in accordance with said time estimatesand an estimated velocity of said tag relative to said interrogator tocoherently integrate said return signals to locate a position of saidtag.
 2. The interrogator as recited in claim 1 wherein said receiver isa correlating receiver.
 3. The interrogator as recited in claim 1further comprising a transmitter configured to code an interrogationsignal for said tag.
 4. The interrogator as recited in claim 1 whereinsaid processor employs a fast Fourier transform to coherently integratesaid return signals.
 5. The interrogator as recited in claim 1 furthercomprising a controller configured control an operation of saidreceiver, said sensing module and said processor.
 6. The interrogator asrecited in claim 1 further comprising a data logger configured to logsaid time estimates and said position of said tag.
 7. The interrogatoras recited in claim 1 wherein said processor is configured to coherentlyintegrate said return signals by constructing a phase trajectory of saidreturn signals based coordinates of a position of said interrogator andcoordinates of an hypothesized location of said tag.
 8. The interrogatoras recited in claim 1 wherein said tag is a radio frequencyidentification tag.
 9. The interrogator as recited in claim 1 whereinsaid return signals comprise inphase and quadrature sample data of saidtag stored in memory for a duration nominally equal to an expectedresponse duration of said return signals.
 10. The interrogator asrecited in claim 1 wherein said tag is configured to move relative tosaid interrogator.
 11. A method of operating an interrogator,comprising: receiving return signals from a tag; providing timeestimates associated with said return signals; and coherentlyintegrating said return signals by employing synthetic aperture radarprocessing on said return signals in accordance with said time estimatesand an estimated velocity of said tag relative to said interrogator tolocate a position of said tag.
 12. The method as recited in claim 11wherein said receiving comprises performing correlation on said returnsignals.
 13. The method as recited in claim 11 further comprising codingan interrogation signal for said tag.
 14. The method as recited in claim13 further comprising sending said interrogation signal to said tag. 15.The method as recited in claim 11 wherein said coherently integratingsaid return signals employs a fast Fourier transform.
 16. The method asrecited in claim 11 further comprising logging said time estimates andsaid position of said tag.
 17. The method as recited in claim 11 whereinsaid coherently integrating said return signals comprises constructing aphase trajectory of said return signals based on coordinates of aposition of said interrogator and coordinates of an hypothesizedlocation of said tag.
 18. The method as recited in claim 11 wherein saidtag is a radio frequency identification tag.
 19. The method as recitedin claim 11 wherein said return signals comprise inphase and quadraturesample data of said tag stored in memory for a duration nominally equalto an expected response duration of said return signals.
 20. The methodas recited in claim 11 wherein said tag is moving relative to saidinterrogator.